Vertical Clearance

The adopted criteria provide vertical clearance values for the various
highway functional classifications (Table 19). These criteria are set
to provide at least a 1-foot differential between the maximum legal vehicle
height and the roadway, with additional allowances for future resurfacing.
These clearances apply to the entire roadway width (traveled way and shoulders).
A formal design exception is required whenever these criteria are not
met for the applicable functional classification.

Clarifications

The specific standards for vertical clearance adopted for the Interstate
System maintain its integrity for national defense purposes. On
Interstates, the clear height of structures shall not be less than 16
feet (4.9 meters) over the entire roadway width, including the useable
width of shoulder. In urban areas, the 16-foot (4.9-meter) clearance shall
apply to at least a single routing. On other urban Interstate routes,
the clear height shall not be less than 14 feet (4.3 meters). A design
exception is required if this standard is not met. Exceptions on
the Interstate must also be coordinated with the Military Surface Deployment
and Distribution Command Transportation Engineering Agency of the Department
of Defense.

Substantive Safety

The adverse effects of structures with insufficient vertical clearance
are obvious (see FIGURE 24). Impacts to low bridges create risk
for the driver of the vehicle, others on both roadways, and in extreme
situations can result in closure of the bridge for lengthy periods and
necessitating costly repairs.

FIGURE 24

Interstate closure after an impact with a bridge.

Figure 24 is a photo showing damage caused to the underside of an interstate
bridge as a result of being struck by a vehicle that was too tall to pass
underneath. Pieces
of concrete are scattered over the roadway, completely closing that direction
of the interstate.

Summary

Table 20 summarizes the potential adverse impacts to safety and operations
of a design exception for vertical clearance.